I am going through some of my lenses to check their AF on an optical bench I have at work (I'll be glad to post about that if needed) and I had a question regarding the default versus saved value.

As far as I can tell (and correct me if I am wrong) the values operate as such:

Default value: ANY lens mounted to the body will undergo this AF fine tune.
Saved value: Specific lenses saved in the list will undergo their respective AF fine tune.

I understand these two values aren't additive... i.e. a default value of "-20" and a saved value of "-20" will not yield -40 units of adjustment.

I can't understand why one would use a default value at all... Can anyone speak towards the default value parameter?

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As an aside, does anyone know if AFS lenses respond better to this feature over AFD lenses?

Finally, I am assuming I am to calibrate the AF within the range I usually operate... for instance if I use my 35 f/2 mainly in ranges within 1-2 meter, I will set up my target at 1m and not 25cm, knowing that I have to be outside of the MFD.

The default value is useful when the body's AF system itself is misaligned and so all lenses mounted on it are off by a certain bias. Btw, Marianne Oelund over at dpreview recently discovered that the scale between the default and specific AF tune values is actually different. You can read about it here: http://forums.dpreview.com/forums/post/42121375

Hmmm... that's interesting. But if I am thinking clearly, this technique of using the different scales is only useful if all of your lenses are off the same amount?

I am more confused now...

In her example (a,b) she changed nothing but the default value from -15 to +15 and noticed an ~8" change in focus plane. When she went to (c,d) she kept this default value but left the saved value at 0 and noticed no difference between -15default and +15default.

So I am confused what the default value does if it doesn't change anything when AF fine tune is active?

SebastianS wrote:
Hmmm... that's interesting. But if I am thinking clearly, this technique of using the different scales is only useful if all of your lenses are off the same amount?
It's useful when you want to apply the same adjustment to all lenses which you haven't entered a lens-specific adjustment value for. Again, for misalignments caused by the body itself.

SebastianS wrote:
In her example (a,b) she changed nothing but the default value from -15 to +15 and noticed an ~8" change in focus plane. When she went to (c,d) she kept this default value but left the saved value at 0 and noticed no difference between -15default and +15default.
So I am confused what the default value does if it doesn't change anything when AF fine tune is active?

For C,D she changed the default value while the lens-specific value was active, which produced no change. This is expected and I presume the experiment was done to just verify that the default value has no affect on lenses with saved values.